Joachim Römelt

Prediction and interpretation of collinear reactive scattering resonances by the diagonal corrected vibrational adiabatic hyperspherical model

One-dimensional single-channel scattering calculations on an effective potential are used to discuss resonances in collinear reactions and to predict their energetic location almost quantitatively. The effective potential includes the vibrationally adiabatic potential and the adiabatic diagonal correction in Delves mass-weighted polar coordinates (hyperspherical coordinates). This diagonal corrected vibrational adiabatic hyperspherical (DIVAH) model is tested for a variety of reactions (H + H2, H + MuH, I + HI, F + H2) and a detailed discussion of the effective potential and its physical implications is given.

The collinear F + H2 reaction evaluated by S-matrix propagation along delves' radial coordinate

Abstract The method of S-matrix propagation along Delves radial coordinate r is used to evaluate exact quantum-mechanical reaction probabilities for the collinear F + H2 reaction. The mass asymmetry induces important crossings and avoided crossings of the vibrational energies along r. The results are in good agreement with the standard references.

Calculation of wavefunctions and frequencies for noninfinitesimal vibrations: Comparison of various methods using ab initio CI potential curves

Abstract Three computational methods employing different means of representing molecular potentials are used to obtain wavefunctions and energy levels for noninfinitesimal vibrations. An interpolation scheme based on a cubic spline fitting procedure is introduced to supplement the CI energy results obtained explicitly in actual calculations. As long as a representative set of potential points is available it is found that the results of all three methods (two of variational and one of numerical integration type) are quite consistent, for both bending and stretching vibrations of ground and electronically excited states (examples for HCN and O 2 are considered). In addition a comparison of one- and two-dimensional bending vibrational treatments is made.

An SCF and MRD-CI study of the ground and excited states of the He + H2 system. I. Calculated potential surfaces

Abstract A series of SCF and MRD-CI calculations is carried out to describe the potential surfaces arising from the collinear and orthogonal approach between an atom and an H 2 molecule in its X 1 Σ g + , b 3 Σ u + , B 1 Σ u + , a 3 Σ g + , c 3 Π u and C 1 electronic states, respectively. In addition to the Heue5f8H 2 separation the distance between the two hydrogen atoms in the H 2 system is sys varied throughout the approach of both fragments. All calculations employ a set of 45 contracted cartesian gaussian functions; error bounds are discuss It is found that the form of the potential surfaces can best be explained by a combination of both the theory of long-range forces and the theory of ch the dispersion forces become stronger with the degree Of H 2 excitation (i.e. increased polarizability) while the effects which are more attributabl bonding enhance or counteract this trend depending on the nature of the individual state. The change in the Hue5f8H separation upon the He( 1 S) a quite marked; in the orthogonal approach a definite decrease in the Hue5f8H distance is seen upon He interaction while the opposite trend is observe collinear arrangement of atoms, i.e. effects which are important in vibrational energy transfer processes. The potential surfaces should present a basi general He + H 2 collision studies extended to excited states; they will be employed in part II for the theoretical study of the observed quenching the B 1 Σ u + → X 1 Σ g + fluorescence by He atoms, as well as related energy transfer processes.

Ab initio MRD CI calculations for the electron spectrum of the C3 radical

Abstract Calculations using the MRD CI method are reported for the ground and low lying excited states of C 3 . Transitions from the 3σ u , 4σ g and 1π u MOs into 1π g are considered, as well as the 1π u → 3s Rydberg species and the corresponding ionization, and good agreement with experimental data is obtained where comparison is possible. Potential curves calculated for the ground and (1π u → 1π g ) 1 Σ + u excited state are discussed.

Ab initio Cl calculation of the effects of Rydberg-valence mixing in the electronic spectrum of the HF molecule

Experimental studies of the HF molecular spectrum have heretofore been unable to arrive at a suitably consistent theoretical assignment for the various measured band systems therein. To aid in this pursuit a series of ab initio Cl calculations in an AO basis containing 40 contracted gaussians has been carried out to an accuracy which is close to the full Cl limit as a result of the use of energy extrapolation techniques described elsewhere. In addition to obtaining generally quite good agreement with experimental spectroscopic constants including the dissociation energy, this treatment allows for a careful description of the change in composition of the HF ground state from ionic to covalent character as molecular stretching occurs, as well as a good representation of the upperB1Σ+ state with which it undergoes a strongly avoided crossing. The repulsive branch of the latter potential energy curve is shown to intersect the Rydberg1Σ+ manifold at relatively short bond distances, leading to a series of mixed valence-Rydberg states which are ultimately responsible for the large deviations from normal Rydberg series which are observed experimentally. Similar crossings of σ→π* and σ→σ* valence states with Rydberg species of3,1π and/or3Σ+ symmetry are calculated to result in heavy mixing over only a relatively short range of bond distances, and thus do not produce the same magnitude of perturbations as do the1Σ+ states. Finally the calculated potential curve for the parentX2π ionic state for such Rydberg species also proves to be quite consistent with known structural data, giving independent evidence for the overall high level of accuracy of the theoretical treatment employed in the present work.

Ab initio MRD-CI study of the rydberg states of methylene

Abstract Potential curves have been calculated for the low-lying Rydberg states of CH 2 as well as for a number of its valence-shell species by employing the ab initio MRD-CI method. The first Rydberg transition is found to occur with a vertical energy of 6.38 eV (1b 1 → 3s), but the corresponding upper state is believed to be strongly predissociated since it correlates directly with the CH( 2 II) + H( 2 S g ) ground state fragments at lower energy. The assignment of the first observed Rydberg transition at 8.757 eV by Herzberg as 1b 1 → 3dπ is confirmed almost quantitatively in the calculations, while the corresponding minimum 1P value is computed to be 10.21 eV compared to the experimental result of 10.3 ± 0.1 eV. The dissociation energy of methylene in its ground state is calculated to be 4.47 eV, and this result also fits in well with experimental evidence, which determines a lower limit for this quantity of D 0 > 4.23 eV. Finally, it is found that none of the Rydberg states nor any of the higher-lying valence-shell species of methylene are of sufficiently low energy to play a significant role in the experimental determination of the 1 A 1 - 3 B 1 splitting of this system.

An SCF and MRD-CI study of the ground and excited states of the He + H2 system. II. Quenching of HD (1Σ+u) fluorescence and other energy transfer processes

Abstract Ab initio MRD-CI calculations reporeted in a previous study are employed to explain the origin of various types of interactive phenomena observed experimentally for mixtures of HD and He in electronically excited states. With the help of additional potential surface calculations it is concluded that the HB B 1 Σ + u fluoresence quenching in the presence of He reported by Fink, Akins and Moore arises because of an avoided corssing witht eh E 1 Σ + g HD state; this interaction results in a notably more shallow potential well for HD stretch in the B state when He is present (collinear approach) than when it is not. Other energy transfer processes involving excited states of the He + H 2 (HD) system are also discussed in light of the present calculations.

Franck-Condon matrix elements for bound-continuum vibrational transitions calculated by numerical integration and basis set expansion techniques

Franck-Condon factor distributions for bound-to-continuum transitions of one-dimensional vibrational states are calculated by a) using numerical integration, b) employing a finite number of square integrable harmonic oscillator functions. The methods are generally applicable to any kind of bound or repulsive potential involved. Results are presented and compared to model potential calculations previously reported by Krüger [1].